This invention relates to a process for preparing microparticles comprising a solid organopolysiloxane. More particularly, this invention relates to the preparation of microspheres and microcapsules using ultraviolet radiation to convert dispersed discrete entities comprising certain fluid organopolysiloxane compositions to microcapsules and microspheres comprising a solid organopolysiloxane.
Microencapsulation, a means for isolating materials in small containers for subsequent release under controlled conditions, is a relatively new and active art. In the approximately twenty-five years of its commercial life span, however, the microencapsulation art has disclosed only a few methods of making microcapsules, each of which can be conveniently categorized as either a chemical method or a mechanical method. The present invention can be classified as a chemical method of preparing microparticles inasmuch as it comprises crosslinking a liquid organopolysiloxane to the solid state to form the microcapsule.
Organopolysiloxanes have been used in chemical processes for forming microcapsules; however, these methods are very limited in scope. For example, Burzynski et al., U.S. Pat. No. 3,257,330 disclose a process for preparing colored gel particles by hydrolyzing organotrialkoxysilanes in an acidic aqueous medium to form a soluble hydrolyzate and thereafter adding an organic dye and heating the solution until insoluble, hard gel particles containing the dye are formed. This disclosure is limited to a process using heat in an acidic medium and would be of little value for microencapsulating heat-sensitive and/or acid-sensitive materials. Breen et al., U.S. Pat. No. 3,551,346 disclose a process for preparing dual wall capsules. An inner wall is formed by a reaction of a siloxane dissolved in a core material, and an alkaline silanolate, dissolved in an aqueous phase. Subsequently, a non-silicone, outer wall is formed by the well-known coacervation process to provide increased durability to the microcapsule.
Compared to microencapsulation, the preparation of microspheres of solid organopolysiloxane appears to be a non-existent or little-practiced art; however, the method of Burzynski et al., discussed above, would seem to be adaptable for preparing hard gel particles, by merely omitting the dye.
A process for providing microcapsules comprising a durable organopolysiloxane exterior and utilizing neutral, room-temperature conditions is desired for encapsulating acid-, base-, or heat-sensitive material. A process for providing elastomeric or resinous microparticles comprising a solid organopolysiloxane is also desired to provide microparticles of varying strength and utility.
Ultraviolet radiation has only recently been used to form microcapsules. Japanese Pat. No. 52-43779 provides a method for producing microcapsules using a photosensitive resin and ultraviolet radiation. The photosensitive resin comprises a monomer or oligomer having two or more photosensitive groups which will undergo a polymerization reaction under the influence of light energy. Examples of photosensitive groups disclosed therein, which undergo said polymerization reaction, include acryloyl, vinyl ether, vinyl thioether, vinyl ether, vinyl linked to benzene, N,N-vinylalkylamino, allyl, acrylamide, 1,2-alkylene oxide and acetylenyl.
Organopolysiloxane compositions which are curable by ultraviolet light are known from Warrick, Canadian Pat. No. 653,301; Berger et al., U.S. Pat. Nos. 3,726,710; Gant, 4,064,027; Viventi, 3,816,282; Michael et al., 3,873,499; Bokerman et al., 4,052,529 and Gordon et al., U.S. Application No. 856,693, filed on Dec. 2, 1977, now U.S. Pat. No. 4,107,390, issued Aug. 15, 1978, and assigned to the assignee of this invention. These inventions are directed to the preparation of various articles such as silicone rubber, conformed coatings for electronic articles and paper coatings for adhesive release; however, there is no indication that these silicone compositions bearing silicon-bonded olefinic groups may be cured by ultraviolet radiation in the dispersed state or that they may be used to prepare microparticles, such as microspheres and microcapsules.